Your Fisher grade classifies the amount and pattern of subarachnoid blood visible on the initial CT scan, which directly correlates with the risk of delayed cerebral vasospasm. Grade 1 (no blood detected on CT) carries the lowest vasospasm risk at approximately 21%. Grade 2 (diffuse thin layers of blood less than 1 mm thick) has a vasospasm risk of about 25%. Grade 3 (localized thick clot or layers of blood 1 mm or greater) carries the highest vasospasm risk at approximately 37%. Counterintuitively, Grade 4 (intracerebral or intraventricular hemorrhage with diffuse or absent subarachnoid blood) has a somewhat lower vasospasm risk (~31%) than Grade 3, though it may carry worse overall outcomes due to the parenchymal or ventricular blood.

Vasospasm typically occurs between days 3 and 14 after SAH, peaking around days 7–10. Your Fisher grade helps determine the intensity of vasospasm surveillance required during this critical window.

Use the Fisher grade when evaluating the initial non-contrast CT scan in a patient with confirmed subarachnoid hemorrhage. It should be assigned as soon as imaging is reviewed, ideally in conjunction with the Hunt & Hess clinical grade, to provide a comprehensive assessment of SAH severity and expected complications.

The Fisher grade is particularly valuable for guiding vasospasm prevention and monitoring strategies. Higher grades (especially Grade 3) warrant aggressive monitoring with transcranial Doppler ultrasonography, nimodipine prophylaxis, and potentially CT angiography if vasospasm is suspected. It is routinely used in neurosurgical and neurointensive care documentation and in research protocols studying SAH outcomes.

The original Fisher scale has been criticized for the paradoxical lower vasospasm risk in Grade 4 compared to Grade 3 and for its limited granularity. The Modified Fisher scale (Claassen et al., 2001) was developed to address these issues by separately scoring the presence of thick subarachnoid blood and intraventricular hemorrhage, providing better vasospasm prediction.

The Fisher grade depends on CT image quality and timing. Very early CT scans (within hours of ictus) may underestimate blood volume, while delayed scans may show blood redistribution or clearance. Inter-observer agreement is moderate, as the distinction between thin and thick subarachnoid blood (the critical 1 mm threshold) can be subjective. The scale also does not account for other vasospasm risk factors such as patient age, smoking history, hypertension, or clinical grade at presentation.